The present invention relates to an image forming apparatus such as digital copiers, laser printers, and the like in which image recording is carried out by scanning the surface of a photoreceptor employing laser beams.
In electrophotographic digital copiers, etc., technology is widely employed in which an electrostatic latent image is written on the surface of a photoreceptor drum, etc., while deflecting laser beams employing a polygonal mirror. The photoreceptor drum of a copier is rotated so that the line speed matches to the conveyance speed of a recording sheet. In digital copiers, the speed of rotation of the polygonal mirror is fixed in accordance with the rotational speed of the photoreceptor drum and pixel-recording density.
Furthermore, when the polygonal mirror is stably rotated at high speed, recording speed and image quality of the digital copier are improved. Accordingly, technology is known in which the rotation shaft is integrated with the polygonal mirror; further, an air bearing, which is one kind of dynamic pressure bearings, is employed on the rotation shaft so as to avoid direct contact of members of the rotation shaft and the bearing each other, allowing the polygonal mirror to be stably rotated at high speed.
Now, because automation and diversification are required to meet requirements for recording sheet output, digital copiers have been increasingly demanded which enable the presentation of a variety of image-processing functions. However, at present, no digital copiers provide the image forming mode in which the speed of rotation of the polygonal mirror is varied.
For example, in analog copiers, when copying is carried out employing thick paper sheets, image forming operation is carried out by decreasing the line speed in order to obtain high quality images. However, in digital copiers, when the line speed is changed, variation in the speed of rotation of the polygonal mirror is required. Accordingly, it has been extremely difficult to provide the image forming mode which outputs good images to thick paper sheets.
The following apparatus has been known in which an electrophotographic image forming apparatus is subjected to mere variation of the speed of rotation of the polygonal mirror. In recent years, an electrophotographic apparatus has been introduced which is employed as a digital copier and a laser printer in one unit. This apparatus is termed a composite apparatus. In such composite apparatuses, the line speed is varied when employed as a digital copier and when employed as a laser printer, and thus the speed of rotation of the polygonal mirror is altered.
Generally, a polygonal mirror is very light and friction caused on the bearings is very small. In an image forming apparatus which is constituted in such a way that the decrease in the rotational speed of the polygonal mirror, which continues to rotate due to inertia is waited, the time until the rotational speed of the polygonal mirror decreases to the predetermined level can be quite long. Particularly, in a high speed apparatus which results in high productivity of image output, a decrease in friction is achieved employing air bearings so as to be in no contact of the polygonal mirror with the portion of the driving source. Accordingly, when the natural decrease in the speed of rotation of the polygonal mirror is waited, the waiting time becomes very long compared with the bearing in which the polygonal mirror is in contact with the portion of the driving source and, for example, several tens of seconds are required for the subsequent image formation. Thick paper sheets may be employed in cases, for example, when a cover is prepared employing a copied sheet of thick paper for copied sheets of normal paper or copied sheets of thick paper are employed as partition sheets and are bound, employing a finisher. In this case, switching from normal paper to thick paper is complicated. If switching from normal paper to thick paper takes several tens of seconds, a long time is required to complete continuous copying operations which decrease the imaging productivity. In addition, when employed upon switching the resolution, during switching, the speed of rotation of the polygonal mirror is occasionally decreased and the waiting time becomes longer.